Image suppression system



Patented Sept. 28, 1937 PATENT OFFICE macs sorrasssron sys'rsm Wladimir J. Polydorofl, wumette'm, assignor to Johnson Laboratories, Inc. Chicago, Ill., a

corporation of Illinois Application February 17, 1936, Serial No. 64,299

2 Claims.

The invention relates to high-frequency circuitsand more particularly to those employed in receiving systems of the superheterodyne type,

' wherein the signal passes through several cir- 5 cults tuned to the signal frequency, is modulated with locally produced oscillations, is demodulated and then furtheramplified at an intermediate frequency, and is finally demodulated to produce an audio-frequency current and is then rendered audible. More particularly, the invention relates to circuits intended to suppress certain undesired responses in superheterodyne receivers.

When a radio receiver of the superheterodyne 16 type is tuned to a desired signal, a spurious response may beobtained from a second signal which is higher (or lower) in frequency than the desired signal by twice the intermediate frequency of the receiver. The undesired signal is 20 commonly called the image-frequency signal, and its frequency is called the image frequency. Thus for every desired signal there is a corresponding image-frequency signal which, if present and of sufficient strength, may cause an un- 25 desired response. For example, if a particular.

desired signal has a frequency of 600 kilocycles and the intermediate frequency of the receiver is.450 kilocycles, the image-frequency signal will have a frequency of 600+(2x450) or 1500 kilo- 30 cycles. The ratio of the input voltage of image frequency required to produce a given output, to the signal voltage required for the same output, is called the image-frequency ratio.

One of the objects of the invention is to provide a simple and effective means for increasing the image-frequency ratio of a' superheterodyne receiver. Another object of the invention is to provide an image-suppression arrangement for a superheterodyne receiver which has no appre- 40 ciable eifect upon the amplification of the receiver at the frequency of the desired signal.

So-called trap circuits similar in some respects to the image-suppression circuits herein disclosed were proposed and had been the subject 45 of extensive experiment as early as 1922. These arrangements, however, were very unsatisfactory, not only because of their inherent low emciency at any frequency, but additionally because their efliciency was far from constant,- so that 50 while the undesired signal was partially excluded at some frequencies, it was not excluded in appreciable degree at other frequencies. Moreover, in

some forms, these early arrangements produced a serious and highly undesirable reduction in the 55 gain of the receiver at-the frequencies of the desired signals. 7

It is a further object of my invention, therefore, to provide circuit'arrangements for successfully rejecting undesired signals, whether of im- 9 age-frequency, or of any other undesired fre-.

quency to which a radio receiver might otherwise respond, such for example as a very strong signal from a nearby or powerful transmitting station.

In a superheterodyne receiver, that portion of the system through which the signal passes without a, change in frequency usually includes an antenna circuit and one or two tuned circuits with or without a thermionic amplifier tube, and is commonly called the pre-selector.. One method of increasing the image-frequency ratio is to employ, in the pre-selector, additional resonant circuits tuned to the signal frequency so that a satisfactory degree of attenuation of the undesired image-frequency signals is obtained. Such an arrangement, however, will usually cause a serious decrease in the gain between the antenna and the first thermionic tube, with a corresponding decrease in the signal-to-noise ratio of the receiver.

According to the present invention, a permea bility-tuned image-suppression circuit is employed which, while materially reducing'the response to undesired image-frequency signals, does not appreciably reduce the gain between the antenna and the first thermionic tube. The image-suppression circuit is similar-to the fa.- miliar wave-trap or signal-absorbing circuit and is suitably coupled to the antenna circuit of the receiver. The efflciency of such a circuit in rejecting undesired signals is directly dependent upon the dynamic resistance (L/RC) of the circuit.

' Permeability-tuned resonant circuits are particularly adapted for this use not only because they are inherently much more emcient than a circuit tuned by a variable capacitor over the same frequency range, but additionally because L their dynamic resistance remains practically constant throughout their tuning range. It is possible, therefore, to build an image-suppression circuit in accordance with the present invention which will have substantially uniform suppression throughout the entire tuning range. Permeability-tuned resonant circuits of the type herein contemplated are disclosed in my United States Patent No. 1,978,600, issued October 30, 1934.

Because of the high selectivity of the imagesuppression circuit, the resonant frequency of which is maintained at a. value higher than the frequency of the desired signal by twice the' intermediate frequency, it produces no appreciable attenuation of the desired signal. This-result may be secured not only in the standard broadcast frequency range, but also at the higher radio frequencies, as for example, between 5 and 20 the antenna and the first thermionic tube is relatively small, so that any reduction of the antenna gain caused by the use of additional selective circuits would be prohibitive.

The invention will be better understood if the following description is read by reference to the accompanying drawing, in which:

Fig. 1 shows the application of the invention to a receiver in which the antenna circuit is directly coupled to the first variably tuned circuit;

Fig. 2 shows the application of the invention to a receiver in which the antenna circuit is inductively coupled to the first variably tuned circuit;

Fig. 3 shows the application of the invention to a receiver employing a doublet antenna system which is inductively coupled to the first variably tuned circuit; and

Fig. 4 shows an alternative method of applying the invention to a receiver in which the antenna circuit is capacitively coupled to the first variably tuned circuit.

Referring to Fig. 1, the antenna circuit includes winding I, capacitor 2 and variable inductor 3.

Inductor 3, consisting of winding 3a and relatively movable core 31), is shunted by-capacitor 4, to form a resonant circuit whose frequency is adjusted by relative movement of the winding and core to correspond with that of the desired signal. A similar resonant circuit consisting of variable inductor 5 and capacitor '6 is inductively coupled to Winding l and its frequency is similarly adjusted to be higher than the frequency of the desired signal by twice the intermediate frequency of the receiver. This relationship is preferably maintained by simultaneous movement of the magnetic cores 3b and 5b of inductors 3 and 5.

The circuit 5-6 is tunable over a frequency range whose ratio of maximum frequency to minimum frequency is less or greater than that of circuit ,34, the difference in range depending upon the intermediate frequency of the receiver and upon the frequencyrange of circuit 3-4. Since the circuit 56 is not shunted by the input capacitance of a thermionic tube, it is possible to employ a higher -value of inductance in this circuit, thus providing better rejection of undesired signals.

For the standard broadcast band of frequencies between 540 and 1600 kilocycles, the permeabilitytuned variable inductors 3 and 5 may be identical with the exception that in inductor 5 a simple plug core is employed, thus adapting it to cover a narrow range of frequencies, whereas in inductor 3 a compound core having an internal plug portion and an external shell portion is employed, to adapt this circuit to cover a wide range of fre quencies. At the upper radio frequencies, however, as for example between 5 and 20 megacycles, it is convenient to employ identical cores and to insert padding inductance in the circuit 5-6 in order to restrict the tuning range of this circuit. Capacitor 6 may be adjusted, with the core 5b entirely out of winding 5a. to obtain the desired highest resonant frequency.

Fig. 2 differs from Fig. 1 only in the manner in which the antenna circuit is coupled to the first variably tuned circuit of the receiver. In this figure the antenna circuit includes winding l and an additional winding 1, which is inductively coupled to inductor 3. In the arrangements of Figs. 1 and 2, circuit 56, which is tuned to the image frequency, absorbs signals of image frequency which are present in the antenna circuit, and substantially prevents them from reaching cir-- cuit 3-4 and subsequently the remainder of the receiver, not shown.

In Fig. 3, the antenna circuit includes winding I, which is inductively coupled to inductor 3, and two windings I, which are wound in opposite directions and inductively coupled to inductor 5. Thus image-frequency signals picked up by either portion of the doublet antenna are efiectively absorbed and prevented from reaching the circuit 3-4. This embodiment of the invention is especially suitable for use at the higher radio frequencies, as for example between 5 and 20 megacycles.

In Fig. 4, the antenna circuit includes inductor 5 and choke coil 8. The antenna circuit is coupled to the circuit 3-4 by means of capacitor 2. Since the image-suppression circuit 5-6 is included in the antenna circuit, rejection of undesired image-frequency signals is very good with this arrangement. In order to prevent circuit 56 from affecting the tuning of circuit 3-4, however, choke coil 8 must have high impedance and capacitor. 2 must be relatively small.

It will be understood that it is within the scope of the invention to employ tuning by capacitance variation instead of permeability tuning in the circuit 3-4, using any suitable means for obtaining simultaneous operation of the variable capacitor and the moving core of inductor 5 in the image-suppression circuit.

It remains to be pointed out that in operation the image-suppression circuit in any of my figures is tuned, either separately or synchronously with the signal-frequency circuits of the receiver, to. the frequency of the undesired signal, to which it ofiers a very high impedance.

Having thus described my invention, what I claim is:

1. In asuperheterodyne'radio receiver includin an input vacuum tube having a grid, a doublet antenna, an input circuit connected between the twoportions of said doublet antenna and including two co-axial and series opposed windings and a third winding connected in series between said co-axial windings and having a mid-tap through which said antenna is connected to ground, a resonant circuit coupled to said third winding and including a coil and a capacitor each adapted to have high ,efliciency within the range of image frequencies of said receiver, and a movable compressed comminuted ferro-magnetic core in the field of said coil for tuning said resonant circuit over said range of image frequencies, whereby said resonant circuit is adapted to effectively prevent an image-frequency signal from being impressed upon said grid.

2. In a radio receiver including an input vacuum tube having a grid, a doublet antenna, an input circuit connected between the two portions of said doublet antenna and including two co-axial and series opposed windings and a third winding connected in series between said co-axial windings and having a mid-tap through which said antenna is connected to ground, a resonant circuit coupled to said third winding and including a coil and a capacitor each adapted to have high eificiency within the range of frequencies to which said receiver is intended to respond, and

a movable compressed comminuted ferro-mag-' netic core in the field of said coil for tuning said resonant circuit over said range of frequencies, whereby said resonant circuit is adapted to effectively prevent an undesired signal of a particular frequency from being impressed upon said grid. WLADIMIR J. POLYDOROFF. 

